Arithmetic amplifiers are extensively used in order to amplify signals in all kinds of circuits. When many such arithmetic amplifiers are concentrated in high density applications, power amplifiers are required which are capable of driving a load using a low DC operating current and a low impedance value.
As the density of semiconductor integrated circuits increases, multiple high-density analog circuits and digital circuits are formed through a single process within a single chip. Analog circuits made using CMOS manufacturing techniques are used in various applications, such as vocal band-pass filters, analog-digital converters, and digital-analog converters.
Three types of power amplifiers are well known in the art and are commonly known as types A, B and AB. In a type A amplifier, the collector current of the output stage is continuously conducting, imposing a constant drain on the power source. When a low-impedance load is driven, output transistors of class A amplifiers must have high power ratings since considerable current and power is developed across the transistor.
In a conventional class B push-pull transistor-pair amplifier, conduction is not maintained over the complete output cycle. Class AB amplifiers are a hybrid of types A and B.
Class A output stages and class AB output stages are limited in their output voltage swings, as is known in the art. Therefore, when a low impedance load is driven, the transistors of the output stage must be large in size. Conventional class B output stages have the advantage that output voltage swing is higher than comparable class A and class AB output stages. But Class B amps have the disadvantages that crossover distortion is increased, and that control of DC operating current becomes difficult. These disadvantages render known Class A, B and AB amplifiers unsuited to CMOS applications requiring the amplifier to drive a low-impedance load at low DC operating current.